Apparatus and method for material processing using a transparent contact element

ABSTRACT

A method of preparing an apparatus for material processing by generating optical breakthroughs in an object. The apparatus includes a variable focus adjustment device. A contact element is mounted to the apparatus, the contact element has a curved contact surface having a previously known shape. The position of the contact surface is determined prior to processing the object, by focusing measurement laser radiation near or on the surface by the variable focus adjustment device, and the focus position is adjusted in a measurement surface intersecting the expected position of the contact surface. Radiation from the focus of the measurement laser radiation is confocally detected. The position of points of intersection between the measurement surface and the contact surface is determined from the confocally detected radiation to determine the position of the contact surface from the position of the points of intersection and the previously known shape of the contact surface.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/941,042, filed on Jul. 12, 2013, which is a continuation of U.S.patent application Ser. No. 12/983,743, filed Jan. 3, 2011, now U.S.Pat. No. 8,502,113, which is a divisional of U.S. patent applicationSer. No. 11/864,500 filed on Sep. 28, 2007, now U.S. Pat. No. 7,863,543,which claims the benefit of German Patent Application No. 102006046370.6filed on Sep. 29, 2006, all of which are incorporated herein in theirentireties by entirety.

FIELD OF THE INVENTION

The invention relates to a material processing apparatus and a method ofpreparing the material processing apparatus for generating opticalbreakthroughs on or in an object.

BACKGROUND OF THE INVENTION

The invention relates to a method of preparing an apparatus for materialprocessing by generating optical breakthroughs in or on an object, whichapparatus comprises a variable, three-dimensionally acting focusadjustment device for focusing pulsed processing laser radiation ondifferent locations in or on the object, wherein a contact element to beplaced on the object is mounted to the apparatus, said contact elementbeing transparent for the treatment laser radiation and comprising, onits side to be placed on the object, a contact surface and, locatedopposite, an entry surface for the processing laser radiation, each ofsaid surfaces having a previously known shape, wherein, prior toprocessing the object, the position of the entry surface or contactsurface with respect to the focus adjustment device is determined byirradiation of measurement laser radiation on said surface, by focusingmeasurement laser radiation near or on said surface by means of thevariable focus adjustment device, with the energy density of the focusedmeasurement laser radiation being too low to generate an opticalbreakthrough, and the focus position of the measurement laser radiationis adjusted in a measurement surface intersecting the expected positionof said surface.

The invention further relates to a material processing apparatus whichcomprises a processing laser providing pulsed treatment laser radiation,an optical device for focusing the processing laser radiation into oronto an object to be processed, such that optical breakthroughs form inthe focus, a focus adjustment device for variable adjustment of thefocus position in or on the object, a contact element to be placed onthe object, which contact element can be mounted to the apparatus andcomprises a contact surface to be placed on the object and, opposite thecontact surface, an entry surface for the treatment laser radiation,each of said surfaces having a previously known shape, and a controldevice for determining the position of the entry surface or of thecontact surface after mounting the contact element and before processingthe object, which control device controls the processing laser and thefocus adjustment device, there being provided also a measurement laserradiation source, also controlled by the control device, for emission ofmeasurement laser radiation, whose measurement laser radiation passesthrough the focus adjustment device and the optical device and causes nooptical breakthroughs in the focus, wherein the control device, in orderto determine the position of the surface, adjusts the focus of themeasurement laser radiation in a measurement surface intersecting theexpected position of the surface.

In material processing, a laser processing device is often employed toscan the regions of the object which are to be processed by a processinglaser beam. The accuracy of positioning the laser beam usuallydetermines the precision achieved in processing. If the laser beam isfocused into a processing volume, exact three-dimensional positioning isrequired. Therefore, for high-precision processing it is usuallyindispensable to keep the object in an exactly defined position to thelaser processing apparatus. For such applications, the aforementionedcontact element is used, because it allows fixation of the object to beprocessed, whereby defined relationships are achievable up to theprocessing volume. In this way, the contact element becomes part of thebeam path of the processing laser radiation.

This is required, in particular, in micro-processing of materials havingonly low linear optical absorption in the spectral range of theprocessing laser radiation. For such materials, non-linear interactionsbetween the laser radiation and the material are used in the art,generally in the form of an optical breakthrough being produced in thefocus of high-energy laser beams. Since the processing effect then onlytakes place in the laser beam focus, exact three-dimensional positioningof the focal point is indispensable. Thus, in addition to atwo-dimensional deflection of the laser beam, an exact depth adjustmentof the focus position is required. The contact element serves to ensureconstant optical conditions, which are known with a certain accuracy, inthe beam path to the object by mechanically coupling the object and thelaser processing device and by also imparting to the object surface ashape having a known optical effect.

A typical application of such a contact glass is the ophthalmic surgerymethod known as femtosecond LASIK, wherein the laser processingapparatus, provided in the form of a therapeutic instrument, focuses alaser beam into the cornea, forming a focus with dimensions on the orderof one micrometer. Then, a plasma forms in the focus, causing a localseparation of the corneal tissue. By suitable sequential arrangement ofthe local separation zones thus generated, microscopic cuts arerealized, e. g. a defined partial volume of the cornea is isolated.

The position of the contact element influences the accuracy of thismethod and, therefore, it is dealt with many times in the literaturewith respect to position determination:

U.S. Pat. No. 6,373,571 discloses a contact lens provided with referencemarks. Said contact lens is adjusted by means of a separate measurementdevice, resulting in a relatively complex configuration. A furtherexample of a contact element is described in EP 1,159,986 A2. Itresembles the contact lens of U.S. Pat. No. 6,373,571, but additionallycomprises a periphery in the form of a holder having line marks whichenable a surgeon to visually position the device. However, suchpositioning is usually too inaccurate.

Since the contact element usually contacts the object to be processed,it is generally required to employ a new, separate adapter for eachobject. This applies, in particular, in ophthalmic applications underthe aspect of sterility. As a consequence, prior to each processingoperation or prior to surgical intervention, respectively, the contactelement has to be mounted to the laser processing device, which is thenprovided e. g. as a therapeutic instrument. WO 03/002008 A1 teaches tomount the contact glass by holding it in a pincer-like means which islocked to the laser processing device. Said locking is effected via acollar guided within a rail. The adapter is pushed in transversely tothe optical axis in a form-locking manner. DE 19831674 A1 describes theuse of a mechanical coupling mechanism wherein a metal rod, attached toa mount of a contact glass at an oblique angle, is held in a sleeve bymeans of a magnet or electro-magnet. However, these attachments do notdefine the position of the contact element with sufficient accuracy.

It is further known from WO 05/039462 A1 to provide a contact glass withposition marks and to add to the laser processing device a confocaldetector unit which, in connection with irradiated measurement laserradiation, allows to detect the position of the marks and to determinethe position of the contact glass therefrom. The accuracy with which theposition of the contact glass is known is thus better than the accuracygiven by the mounting mechanism as described in DE 10354025 A1, forexample, and by the manufacturing tolerances of the contact glass.

WO 04/032810 A2 also pursues the goal of accurately determining theposition of the contact glass. It describes a method or a device of thetype described above. For exact determination of the position of thecontact surface of the contact glass, which surface is pressed onto theeye, said publication suggests to use the effect of the treatment laseron the contact surface. The treatment laser is controlled such that itis focused at a multiplicity of points and emits treatment laserradiation pulses. Those laser beam pulses which are focused on theboundary surface of the contact glass produce an optical breakthrough bya non-linear effect, which manifests itself as a corresponding plasmaspark. Thus, detection of a spark gives an indication that the boundarysurface is located at the present focus position of the treatment laserbeam. The determination of a sufficient number of such points, which maybe arranged in a plane perpendicular to the optical axis of thetreatment laser beam, for example, is then used to determine theposition of the contact glass. In an alternative approach, saidpublication suggests to utilize non-linear effects on the contact glass,which are produced by lower energy levels of the treatment laser pulses,i. e. by energy levels causing no optical breakthroughs. Of course, suchnon-linear effects below the energy threshold for optical breakthroughsoccur only with certain contact element materials. The publicationmentions non-linear effects in the form of the second harmonic of theirradiated treatment laser radiation or white-light radiation. Themeasurement or detection of such radiation poses high chromatic demands,because radiation, e. g., of twice the irradiated frequency has to bedetected. This makes the optical system complex.

In a third approach, the aforementioned publication suggests to detectthe position of the contact surface of the contact glass by means of aninterference arrangement. However, the interference patterns producedthereby are generally not suitable for adjustment or consideration byinexperienced users. They are extremely difficult to evaluateautomatically. The concept of said citation, as far as it is suitablefor application, requires either irradiation of high-energy laserradiation forming optical breakthroughs at the boundary surface of thecontact glass, which is disadvantageous under aspects of radiationprotection, or requires contact glass materials which exhibit anon-linear effect for the processing laser radiation.

In contact elements, the contact surface to be placed on the object isusually manufactured with great precision. Therefore, the aforementionedmethods and devices of WO 04/032810 A2 determine the position of thecontact surface, which may be planar, for example.

Therefore, it is an object of the invention to improve a method ordevice of the above-mentioned type with respect to determining theposition of the contact glass.

SUMMARY OF THE INVENTION

According to the invention, this object is achieved by a method ofpreparing a material processing apparatus, wherein radiation scatteredback or reflected back from the focus of the measurement laser radiationis confocally detected, the position of points of intersection betweenthe measurement surface and the entry surface or contact surface isdetermined by the confocally detected radiation and the assigned settingof the variable focus adjustment device, the above step being repeatedseveral times, if necessary, with a modified, in particular shifted,measurement surface until a determined number of points of intersection,preferably five, have been detected, and the position of said surface isdetermined from the position of the points of intersection and thepreviously known shape of the entry or contact surface.

The object is further achieved by an apparatus of the above-mentionedtype wherein a confocal detector device is provided which confocallydetects radiation scattered back or reflected back from the focus of themeasurement laser radiation and supplies measurement signals to thecontrol device, and the control device determines the position of pointsof intersection between the measurement surface and the entry surface orcontact surface, said control device varying, in particular shifting,the measurement surface, if necessary, in case there are no or too fewpoints of intersection, and determining the surface position on thebasis of the position of the points of intersection and of thepreviously known shape of the entry surface or contact surface.

The contact surface is a suitable surface for position determination. Ifthe geometry of the contact element between the entry surface and thecontact surface is given with sufficient precision, determination of theposition of the entry surface is also conceivable, of course. Inparticular in the case of planar or spherical contact elements, thegeometry between the entry surface and the contact surface can usuallybe manufactured with such precision that the entry surface position canalso be determined for such contact elements. This has the advantagethat there is always a clear discontinuity of the refractive index atthe usually non-contacted entry surface, regardless of whether thecontact element has already been placed on the object.

The invention uses for detection a linear interaction between themeasurement radiation and the surface to be detected (i. e. the entrysurface or the contact surface). It is neither required to focus laserradiation to an energy density which achieves an optical breakthroughnor does the material of the contact glass have to exhibit a non-linearinteraction with the measurement laser radiation below the energythreshold for an optical breakthrough. Also, the optical system of use(e. g. an objective) only has to pick up radiation of the samewavelength as the treatment radiation, which does not give rise to anyadditional (chromatic) demands. Thus, in contrast to the prior art, theconfiguration is not made more complex and more expensive by thedetection function.

The approach according to the invention serves to determine the exactposition of the contact element (also referred to as a contact glasshere). Since the boundary surface of the contact glass is usuallycurved, its shape is commonly described by a coordinate system referringto the contact glass, for example, in the case of a curved contactsurface, to a vertex or to any other marked point of the curvature. Incontrast thereto, material processing or the apparatus for materialprocessing, respectively, is operated with respect to a coordinatesystem referring to the apparatus itself. Now, determining the exactposition of the contact surface or of the entry surface of the contactglass allows the coordinate system of the contact glass or its surfacecurvature to be adjusted or adapted to the coordinate system of theapparatus or of the material processing method or to determine andconsider the offset existing between the two coordinate systems.

The measurement surface is selected such that it intersects the expectedposition of the surface to be detected on the contact element. Theexpected position is previously known because the contact glass isattached to the processing device prior to determining the position. Dueto the thus known geometric conditions, the region of the expectedposition of the surface is predetermined by the tolerances duringmounting as well as the possible variations which may result duringmanufacture of the contact glass or with different types of contactglasses.

By shifting the focus position of the measurement laser radiation in themeasurement surface, points of intersection between the measurementsurface and the surface to be detected are sought. The confocaldetection enables unambiguous detection of the discontinuity of therefractive index which appears at the surface of the contact glass,which is preferably not contacted at this location. If the focus isshifted in the measurement surface, a discontinuity of the refractiveindex occurs every time the focus is located at a point of intersectionbetween the surface to be detected and the measurement surface, whichdiscontinuity results in a corresponding signal during confocaldetection.

The confocal detection of the back-scattered or back-reflectedmeasurement laser radiation advantageously uses the fact that the partof the confocally detected emission radiation which is scattered back ata boundary surface of a transparent medium is significantly higher thanwithin the transparent medium. Due to the spatial filtering occurring inthis case, the confocal detection yields a sufficiently strong signalwhose strength depends substantially on the difference in refractiveindex at the contact surface of adjacent media. Thus, for example, saiddifference is greater for the transition from glass to air than for thetransition from glass to lacrimal fluid. Therefore, when determining theposition of the contact surface, it is preferred to effect determinationof the position before the contact element is placed on the object to beprocessed, e. g. on the cornea of the eye. Since the energy of themeasurement laser radiation does not result in any processing effect, itis also possible, of course, to determine the position of the surfaceafter the contact element has been placed on the object, i. e. has beenwetted with lacrimal fluid in the case of ophthalmic surgery.

Of course, the measurement surface need not be completely covered by thefocus shift. It is absolutely sufficient if enough points, i. e. a(sufficiently) dense path curve, are located in the measurement surface.The condition to be fulfilled is that a sufficient number of points ofintersection for position determination are found. If need be, thenumber of points in the measurement surface can be increased. Further,any two-dimensional manifold will be sufficient whose extent is suchthat it intersects the expected position of the known surface to bedetected.

Therefore, the focus position is preferably shifted along a path curvelocated in the measurement surface. The path line should be selectedsuch that it intersects the sought surface of the contact glass, i. e.contains common points between the measurement surface and the surfacein the expected position of the latter. It is generally sufficient ifthe path line yields at least five discernible points of intersectionbetween the measurement surface and the surface to be detected in caseof a curved surface to be detected.

Based on the position of the points of intersection, the position ofsaid surface is then determined, taking into consideration thepreviously known shape of the entry surface or of the contact surface.For non-spherical surfaces, this may also include determining a tilt ofthe main optical axis of the system.

In the case of a rotationally symmetric surface, fewer points ofintersection will be sufficient. For surfaces which are conventionallyused for spherically curved contact elements, it is preferred that themeasurement surface be located such that it has cylindrical symmetryabout the main optical axis of the processing laser beam, preferably theshape of a cylinder shell or of a circular disk. A cylinder isparticularly favorable, because it has turned out that the z coordinate,i. e. the coordinate along the main optical axis of the processing laserbeam, is subject to particularly great variations with respect tomounting of the contact element and its manufacturing tolerances.Providing the measurement surface as a cylindrical surface has theadvantage that a particularly large area can be covered in the zdirection (along the main optical axis of the measurement or processinglaser radiation). The path line in this cylinder shell can be providedas a spiral, for example. Selecting a sufficient pitch of this spiralwill ensure that a sufficient number of points of intersection betweenthe measurement surface and the (rotation symmetric) surface to bedetected are given. If necessary, i. e. if there are not enough pointsof intersection (see above), the path line can be modified in aniterative manner.

Of course, detection of the surface is facilitated by increasing thepower of the measurement laser radiation. An optimum signal/noise ratiois achieved at a pulse energy of approximately 20 nJ. The intensity ofthe measurement laser radiation which can be applied, in particular, ina pulsed manner, is selected such that no optical breakthrough occurs inthe focus. Therefore, a pulse energy of less than 300 nJ is preferredfor a pulsed measurement laser radiation. This value represents an upperlimit which should not be exceeded so as to avoid optical breakthroughsor other non-linear processing effects on the contact element.

Further, the use of as little measurement laser radiation power aspossible is recommended so as to keep exposure to radiation as low aspossible for users or third parties. Of course, not only the pulseenergy, but also the pulse frequency is relevant in this connection,because they both determine the dose together. For a pulse frequency ofbetween 50 and 500 kHz, an upper limit of 10 nJ, or even 5 nJ, isconveniently advantageous, because at typical wavelengths of between1,000 nm and 1,060 nm, the maximum radiation power which can be focusedon the retina by a viewer's eye lens will then remain within anon-hazardous range.

A processing effect on a transparent object can be achieved by anoptical breakthrough. However, it is also possible to achieve aprocessing effect by repeatedly introducing laser radiation pulses intoa particular volume element, within a determined, narrow time interval,said pulses being below the threshold for an optical breakthrough. Theusual explanation for this effect is that the pulse energy EPULS of thelaser radiation can be accumulated in the volume within a comparativelyshort time and thus leads to a processing effect. In order to avoidthis, the number of laser pulses emitted as measurement laser radiationwithin less than 20 seconds should not exceed a certain number withinthe maximum extent of said volume. Indicating the maximum extension orlength of the path line as D and the pulse frequency as f will yield thefollowing in equation: f<20 Hz*((D/EPULS)*(1 μJ/1 mm))⁴. The maximumextent D therein is that extent of the path line at which the latterwould appear to the observer to be a luminous object, if the observerwere able to perceive the spectral range of the measurement laserradiation (with a corresponding spatial resolution). D should be atleast 1 μm, but not more than 20 mm. These sizes result from thepossible deviations from the expected position of the sought surface.

The aforementioned in equation also takes into account the radiationexposure which the measurement laser radiation should not exceed. Sincethe maximum extent of the object is applied to the power of four in thisin equation, the radiation exposure can be reduced considerably by asuitable selection of the path line, in particular of the lateral extentof the path line.

To provide still further safety for users or other persons, a safetydevice may also be used during position determination, which safetydevice absorbs measurement laser radiation possibly transmitted by thecontact element. For example, a safety cap can be placed on the contactelement, on the side opposite that side where the measurement laserradiation is incident, said safety cap preferably not contacting thecontact surface in order to obtain a discontinuity of the refractiveindex at the contact surface. This is advantageous also for reasons ofsterility. As an alternative or in addition, a covering mechanism may beused which is incorporated in the processing apparatus and is activated,e. g. swiveled into position.

Especially in laser surgery, different contact elements are used in manycases, which differ with respect to the geometry of their input and/orcontact surfaces. One individualizing parameter may be, for example, thecurvature of the surface or the diameter of the contact element. Duringmaterial processing or during operation of the corresponding apparatus,it should be taken into account, of course, which contact element, i. e.which input or contact surface, is currently present. When determiningthe position of the surface, the exact shape of said surface is notknown. However, what is known is that the shape of the surface belongsto a specific group of shapes, because the supply of possible contactelements is limited, of course. Not least in the case ofrotation-symmetric contact elements, but definitely in the case ofspherical surfaces, the positions of the surface's vertex and of thesurface's edge relative to each other provide an unambiguous indicationas to which surface of the limited group of surfaces the contact elementcurrently mounted to the apparatus comprises. Therefore, the position ofthe vertex of the curved surface is preferably determined, as is theposition of the edge of the surface, and the distance or the mutualposition of these structures is used to determine which contact elementor which contact surface is present. This determination may includedetermining the radius of curvature.

As already mentioned, the above-mentioned object is also achieved by acorresponding apparatus. Insofar as method steps carried out whenpreparing such apparatus have been described above or will be describedbelow, such method steps can be carried out, automatically controlled,by a corresponding control device. The intervention of a personeffecting treatment is not required. In particular, the method can becarried out fully automatically, by retrieving a corresponding routinein a control program of the apparatus. This applies equally to variantswhere the object to be processed is not yet in contact with the contactelement and also to variants where the contact element has already beenplaced on the object. Since the measurement laser radiation further hasno processing effect (see above), the method according to the inventionis not of a therapeutic or diagnostic nature, even if it is employed inconnection with ophthalmic surgery.

Of course, the embodiments and features described above or below canalso be employed, in an advantageous manner, individually or in othercombinations not expressly mentioned.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below, by way of exampleand with reference to the drawings, wherein:

FIG. 1 shows a schematic representation of an apparatus for materialprocessing in the form of a processing apparatus for ophthalmic surgery;

FIG. 2 shows an enlarged schematic representation of a contact glassused in the apparatus of FIG. 1;

FIG. 3 shows a further enlarged representation of the contact glass ofFIG. 2, additionally indicating the position of a measurement surface bymeans of which the position of the lower side of the contact glass isdetermined;

FIG. 4 shows a schematic representation illustrating the positiondetermination, said schematic representation corresponding to a partialbottom view of FIG. 3, and

FIG. 5 shows a schematic representation of a further possiblemeasurement surface as well as of a path line located therein, fordetermining the position of the contact glass in the apparatus of FIG.1.

DETAILED DESCRIPTION

FIG. 1 shows a treatment apparatus for an ophthalmic method similar tothose described in EP 1159986 A1 and U.S. Pat. No. 5,549,632. Thetreatment apparatus 1 serves to carry out a correction of an eyesightdefect on the eye 2 of a patient according to the known LASIK method ora similar method. For this purpose, the treatment apparatus 1 comprisesa treatment laser 3 which emits pulsed laser radiation. The pulseduration is, for example, in the femtosecond range, and the laserradiation acts by means of non-linear optical effects in the cornea, inthe manner described above, e.g. by generating optical breakthroughs inthe cornea.

The laser beam 4 emitted by the laser 3 is incident on a scanner 6 whichis realized, in the described schematic embodiment, by two scanningmirrors which are rotatable about mutually orthogonal axes. The scanner6 two-dimensionally deflects the laser beam. Thus, following the scanner6 as well as its subsequently arranged scanning optics 7, a fan-shapedbeam 8 is present, which is adjusted at certain angles with respect to amain optical axis of the direction of incidence, depending on theposition of the scanners 6. After deflection by a beam splitter 9, whichprovides an optical viewfinder for a user, the fan-shaped beam isfocused by a tube lens 10 as well as by an adjustable lens 11 to form afocus which is located at the front portion of the eye 2, e. g. thecornea 18. For each ray of the fan-shaped beam, i. e. for each positionof the scanner 6, a corresponding lateral displacement of the focus isrealized with respect to the main optical axis being present withinactive scanners.

Together with the tube lens 10, the adjustable lens 11 forms projectionoptics which realize a displacement of the focus along the main opticalaxis, i. e. in the so-called z direction. Thus, the combination of thelens 11 and the scanner 6 consequently provides a three-dimensionallyacting focus adjustment device. This focus adjustment device iscontrolled by a control device 17 such that, for example, the knownLASIK method can be carried out using the apparatus 1.

As already mentioned above, in order to achieve the required constantconditions of incidence on the cornea 18 and in order to subsequentlyfix them also in space, a contact glass 19, which will be discussedlater, is placed on the cornea 18.

The treatment apparatus 1 corresponds to the known construction insofaras it is also described in WO 2004/032810 A2. However, in comparisonwith the apparatus described therein, the treatment apparatus 1 has aconfocal detector 12 added to it. The confocal detector 12 isincorporated in the beam path of the incident laser beam 4 prior todeflection of the latter by the scanner 6. Thus, the beam splitter 13 islocated in the resting beam path and has the effect of a color splitterknown from laser scanning microscopy, with a non-spectral splittereffect being possible here as well.

The confocal detector 12 detects radiation, i. e. the radiationscattered back or reflected back in the cornea 18, i. e. in the focusselected by the three-dimensionally acting focus adjustment device, andcouples it out at the beam splitter 13. The radiation to be detectedpasses, in the opposite direction, through the beam path of the laserbeam 4 from the focus to the beam splitter 13.

Pinhole optics 14 as well as a subsequently arranged pinhole 15 causethe desired confocal filtering with respect to the focus in the cornea18, so that only radiation scattered back or reflected back from thefocus passes to the further subsequently arranged detector 16. Thedetector 16 is also connected with the control device 17 via lines (notshown), which control device 17 can assign the signal from the detector16 to the respective focus position by resorting to the correspondingcontrol of the three-dimensional focus adjustment device (scanner 6 andlens 11) and can thus generate an image.

The contact glass 19 used in the apparatus 1 of FIG. 1 is shown,schematically enlarged, as a sectional view in FIG. 2. As can be seen,the contact glass 19 has a planar entry surface 30 and a contact surface20 which is rotation-symmetric in the embodiment example, but isgenerally planar or curved. As disclosed in WO 2004/032810 A2, planarcontact glasses can also be used. Also, the entry surface 30 can becurved. The rotation-symmetric contact glass 19 shown in FIG. 2comprises a vertex 21 which is defined as the point where the opticalaxis of the contact glass 19 passes through the contact surface 20 inthe case of a rotation-symmetric contact surface 20. Of course, in caseof a contact glass 19 having a curved entry surface 30, a vertex (also)exists here. However, by way of example, the construction of FIG. 2 willbe assumed in the following.

As FIG. 3 shows, the curvature of the contact surface 20 is usuallydescribed in a coordinate system (for example, by coordinates of acylinder or of a sphere) relating to the vertex 21. This coordinatesystem is schematically indicated in FIG. 3 and has the referencenumeral 25.

After fixing the contact glass 19 to the treatment apparatus 1, forexample by means of a mechanical system as described in WO 05/048895 A1,the contact glass 19 (and thus also its contact surface 20) has aspatial position that is fixed with respect to the treatment apparatus1, but this position has an inherent tolerance.

Three-dimensional adjustment of the focus is effected in the treatmentapparatus 1 in a coordinate system 21 relating to one of the elements ofthe treatment apparatus 1 which are present during operation, usuallythe scanner 6 or the contact surface of the contact glass. Thiscoordinate system 24 is schematically indicated in FIG. 3. It usuallydoes not coincide with the coordinate system 25 in which the curvatureof the contact glass is described. This is because the main optical axis22 of the incident laser radiation, due to inevitable tolerances whenmounting the contact glass 19 as well as due to the manufacturingtolerances for the contact glass, may usually be shifted and/or tiltedwith respect to the optical axis or the vertex 21, respectively, of thecontact glass 19. There is also usually some uncertainty as to theposition of the vertex 21 in the z direction, i. e. in the longitudinaldirection of the main optical axis 22, because, in particular, thethickness at the center of a contact glass 19 is very difficult tomanufacture within narrow tolerances.

In order to determine the offset between the coordinate systems 24 (ofthe treatment apparatus 1) and 25 (of the contact glass 19), measurementlaser radiation is irradiated through the beam path of the treatmentapparatus 1. The treatment laser 3 is then conveniently used as aradiation source for the measurement laser radiation, because, in theembodiment as presented, the treatment laser 3 can be controlled in anoperating condition in which it can emit pulsed laser radiation with apulse energy resulting in no non-linear interaction, in particular nooptical breakthrough, in the focus, i. e. after passing through theoptical system of the treatment apparatus 1. Suitable attenuators arealso possible. Of course, a separate radiation source for measurementlaser radiation may also be used. However, it is essential that themeasurement laser radiation have a sufficiently precise relationship tothe coordinate system 24. This is particularly easy to ensure if themeasurement laser radiation also passes through the focus adjustmentapparatus, i. e. the scanner 6, the tube lens 10 and the lens 11, i. e.if it is adjusted within the coordinate system 24 of the apparatus. Onlythen can the offset between both coordinate systems be determinedsufficiently exactly.

The measurement laser radiation in the form of a low-energy laser beam 4is now shifted along a path which is located within a measurementsurface 23. The position of the measurement surface 23 is selected suchthat it intersects the expected position of the contact surface 20. Inthe embodiment shown in FIG. 3, a measurement surface 23 is selectedwhich, described in cylinder coordinates of the coordinate system 24, islocated on a constant z coordinate. Thus, the measurement surface 23 isa surface perpendicular to the main optical axis 22. Accordingly, thecoordinates of the curve in the measurement surface 23 of thisembodiment differ with respect to their radial coordinates or theirangular coordinates, but have a constant z coordinate. The intersectionbetween the measurement surface 23 and the contact surface 20 is aclosed path line which is circular in the case of the presentspherically curved contact surface 20, because a spherical section whichdoes not contain the center regularly leads to a small circle. Now, thissmall circle is offset with respect to the main optical axis 22.

This offset is clearly visible in FIG. 4 which shows the view of FIG. 3from below. The offset as well as the radius of the small circle, whichis indicated as the intersecting line 26 in FIG. 4, allows easycalculation of the offset in the coordinate system 25. This applies notonly to spherical contact surfaces, but quite generally also torotation-symmetric contact surfaces, as far as the shape of the contactsurface is known. In the case of a spherical contact surface 20, theradial coordinates and the angular coordinates of the center of thesmall circle on which the points of intersection 26 are located areautomatically the corresponding lateral coordinates of the vertex 21.The z coordinate Za results from the z coordinate Zk of the measurementsurface 23, as well as the radius of curvature R of the contact surface3 and the radius r of the aforementioned small circle by the equation:Za=Zk+R−(R²−r²)^(1/2).

In addition to the aforementioned parameters/structures, FIGS. 3 and 4also show the edge 27 of the contact surface 20. Said edge 27 can alsobe detected by suitably shifting the measurement surface 23 along themain optical axis 22. This merely requires the use of a group ofmeasurement surfaces 23 in order to find the edge 27. For example, it ispossible, also in the case of a contact surface 20 which is selectedfrom a group of possible contact surfaces, to determine from the radialcoordinates of the vertex 21 and the edge 27 that contact surface 20which is actually present. For this purpose, the person skilled in theart can use the difference in radial coordinates between the vertex 21and the edge 27. After determining which contact surface of the knowngroup is present, the aforementioned adjustment of the coordinatesystems 25 and 24 can then be effected with high precision withouthaving to known in advance which of the contact glasses 19 from a groupof possible contact glasses was actually mounted to the apparatus 1.

FIG. 5 shows a further embodiment, wherein a different measurementsurface 23 was used. What applies for all points of the measurementsurface 23 here is that the radius is constant in cylinder coordinatesof the coordinate system 24. The measurement surface 23 is a cylindershell. The path line located within this shell can be provided, forexample, as a spiral 29. The points of intersection 26 (which are notindicated for the sake of a simpler representation in FIG. 5) which theboundary surface 20 has with the measurement area 23 are in turn locatedon a closed path line. In this case, it is easy for the person skilledin the art to determine the position of the contact surface 20. Inparticular, a simple analytical solution can be applied for a sphericalcontact surface.

The invention claimed is:
 1. A method of material processing byproducing optical breakthroughs in or on an eye, the method comprising:using a treatment apparatus comprising a laser source emitting pulsedprocessing laser radiation of femtosecond pulse length and a variablefocus adjustment device for focusing the pulsed processing laserradiation along an optical axis to the eye and for adjusting the focusin three dimensions based on a first coordinate system relating to aposition of an element of the treatment apparatus; describing a geometryof the eye in a second coordinate system relating to a position of acornea of the eye wherein the first coordinate system and the secondcoordinate system differ; detecting a position of the eye by controllingthe variable focus adjustment device to focus measurement laserradiation to the eye; determining an offset between the first coordinatesystem and the second coordinate system, and producing opticalbreakthroughs by focusing the pulsed processing laser radiation todifferent locations in or on the eye by using the apparatus, wherein thevariable focus adjustment device is controlled based on the firstcoordinate system and the offset.
 2. The method as claimed in claim 1,wherein a surface of the eye or a surface being in fixed geometricalrelationship thereto is detected to detect the position of the eye,which surface is a detected surface.
 3. The method as claimed in claim2, wherein the shape of the detected surface and an expected position ofthe detected surface are known previously and the position of thedetected surface is detected by adjusting the focus of the measurementlaser radiation in a measurement surface which intersects the expectedposition, and further by: a) detecting radiation scattered back orreflected back from a focus of the measurement laser radiation; b)determining a position of points of intersection between the measurementsurface and the detected surface from the detected radiation and asetting of the variable focus adjustment device; c) if a pre-determinednumber of points of intersection has not been found, repeating the steps(a) and (b) with modified measurement surfaces until the pre-determinednumber of points of intersection has been found, and d) determining theposition of the detected surface from the positions of the points ofintersection and the shape of the detected surface.
 4. The method asclaimed in claim 3, wherein in step (c) the measurement surface ismodified by shifting the measurement surface used before in steps (a)and (b).
 5. The method as claimed in claim 1, wherein a tilt between thefirst coordinate system and the second coordinate system is determinedalso by measuring a tilt of the eye relative to the optical axis.
 6. Themethod as claimed in claim 1, wherein an energy density of themeasurement laser radiation is below an optical breakthrough thresholdof the eye and the focus position is shifted along a path curve which islocated in the measurement surface.
 7. The method as claimed in claim 3,wherein an energy density of the measurement laser radiation is below anoptical breakthrough threshold of the eye and the measurement surface iscylinder-symmetric to the optical axis.
 8. The method as claimed inclaim 3, wherein an energy density of the measurement laser radiation isbelow an optical breakthrough threshold of the eye and the measurementsurface has the shape of a cylinder shell or of a circular disk.
 9. Themethod as claimed in claim 1, wherein an energy density of themeasurement laser radiation is below an optical breakthrough thresholdof the eye and the measurement radiation is pulsed with a pulse energynot exceeding 300 nJ.
 10. The method as claimed in claim 6, wherein anenergy density of the measurement laser radiation is below an opticalbreakthrough threshold of the eye and the focus position is shiftedalong a path curve which is located in the measurement surface, has amaximum extent D, a pulse energy EPULS and a pulse frequency f for whichf<20 Hz*((D/EPULS)*(1 μJ/1 mm))⁴ holds true.
 11. The method as claimedin claim 6, wherein a path curve is used which is located in themeasurement surface and has a maximum extent D of between 1 μm and 15mm.
 12. The method as claimed in claim 1, wherein an energy density ofthe measurement laser radiation is below an optical breakthroughthreshold of the eye and the measurement laser radiation is generated byswitching pulsed laser radiation source a mode of reduced pulse energyor by arranging or activating an energy attenuator downstream the pulsedlaser radiation source.
 13. The method as claimed in claim 1, whereinthe eye is coupled to the apparatus by a contact element, which istransparent for the processing laser radiation, said contact elementhaving a contact face to be placed onto the eye and, located oppositeand in known geometrical relation to the contact face, an entry face forreceiving the processing laser radiation, both faces having a knownshape, wherein the known shapes are defined in the second coordinatesystem relating to the contact glass and the position of the eye isdetected by detecting the position of the contact or entry face.
 14. Themethod as claimed in claim 13, wherein the contact or entry faceconstitutes a detected surface and is detected by controlling thevariable focus adjustment device to focus measurement laser radiationnear or on the detected surface, an energy density of the focusedmeasurement laser radiation being too low to produce an opticalbreakthrough, and by detecting measurement laser radiation scatteredback or reflected.
 15. The method as claimed in claim 13, wherein theposition of the contact or entry face is detected after the contactelement has been coupled to apparatus, but before the contact face isplaced onto the eye.
 16. The method as claimed claim 15, wherein thecontact face is covered during irradiation of the measurement laserradiation.
 17. The method as claimed in claim 15, wherein the contactface is covered in a contactless manner.
 18. The method as claimed inclaim 1, wherein the eye has a curved surface and the position of avertex of the surface is determined and stored as a reference point. 19.The method as claimed in claim 13, wherein the contact or entry face hasa curved surface and a shape of the surface is previously known tobelong to a group of several different shapes, each known with respectto its exact geometry, and further comprising determining the positionof a surface edge and, based on the relative position of the edge and ofa vertex, determining the shape from the group of shapes which thesurface has.
 20. A treatment apparatus for material processing of an eyehaving a geometry described in a second coordinate system relating to aposition of a cornea of the eye, the apparatus comprising: a lasersource emitting pulsed processing laser radiation of femtosecond pulselength and a variable focus adjustment device for focusing the pulsedprocessing laser radiation along an optical axis to the eye and foradjusting the focus in three dimensions based on a first coordinatesystem relating to a position of an element of the treatment apparatus,wherein the laser source and the variable focus adjustment device areadapted such that optical breakthroughs form at the focus within the eyeand wherein the first coordinate system and the second coordinate systemdiffer; a measurement laser radiation source emitting measurement laserradiation to the variable focus adjustment device for focusing themeasurement processing laser radiation along the optical axis to the eyeand for adjusting the focus in three dimensions based on the firstcoordinate system; a detector device detecting measurement laserradiation scattered back or reflected back from the eye and providingmeasurement signals; a control device controlling the laser source, themeasurement laser radiation source, and the variable focus adjustmentdevice, wherein the control device determines an offset between thefirst coordinate system and the second coordinate system by controllingthe measurement laser radiation source and the focus adjustment deviceto shift the focus of the measurement laser radiation and by detecting aposition of the eye from the signals of the detector device during saidshift, and wherein the control device controls the laser source and thevariable focus adjustment device to material processing of the eye basedon the first coordinate system and the offset.
 21. The apparatus asclaimed in claim 20, wherein the control device detects a surface of theeye or a surface being in fixed geometrical relationship thereto as adetected surface.
 22. The apparatus as claimed in claim 20, wherein thecontrol device shifts the focus of the measurement radiation in ameasurement surface intersecting an expected position of the eye; anddetermines positions of points of the detected surface on the basis ofthe measurement signals, said control device modifying, if apre-determined number of points of intersection has not been found, themeasurement surface until the pre-determined number of points ofintersection have been found, and determines, on the basis of theposition of the points and the previously known shape of the detectedsurface, the position of said surface.
 23. The apparatus as claimed inclaim 22, wherein the control device varies the measurement surface byshifting the measurement surface used before.
 24. The apparatus asclaimed in claim 20, further comprising a contact element for couplingthe eye to the apparatus, which contact element comprises a contact faceto be placed on the eye and, opposite the contact face and in knowngeometrical relation to the contact face, an entry face for receivingthe processing laser radiation, both faces having surfaces of apreviously known shape, wherein the known shapes are defined in thesecond coordinate and wherein the control device detects the position ofthe eye by detecting the position of the contact or entry face.
 25. Theapparatus as claimed in claim 24, further comprising a coveringmechanism which covers the contact element at the contact face forabsorption of transmitted measurement laser radiation.
 26. The apparatusas claimed in claim 20, wherein an energy density of the measurementlaser radiation is below an optical breakthrough threshold of the eyeand the measurement laser radiation is pulsed and the pulse energy isbetween 2 nJ and 300 nJ.
 27. The apparatus as claimed in claim 20,wherein an energy density of the measurement laser radiation is below anoptical breakthrough threshold of the eye and the measurement laserradiation source is constituted by the processing laser radiation sourcebeing controlled in a mode of operation for emission of low-energy laserradiation pulses.